A diversity of linearly reciprocating mechanisms and so on installed in recent years in semiconductor manufacturing equipment, precision machines, industrial robot, assembling machines and so on is designed compact or reduced in construction in favor of energy saving or high energy efficiency. Correspondingly, linear motion guide units built in relatively sliding parts used in the machinery as stated just earlier are also needed compact or downsized in construction while expected to carry heavy loads and make sure of smooth and accurate movement of the slider relative to the guide rail. With conventional linear motion guide units in which the guide rail has bolt holes used to fasten the guide rail to the machine bed and fastening bolts are screwed into the bolt holes to fasten the guide rail to the machine bed, a cap bolt has been fit pressed into a counterbored hole to prevent any foreign matter from creeping into a gap between the bolt hole and the top surface of the guide rail.
In a commonly-owned Japanese Laid-Open Patent Application No. 2002-48 138, recited as patent literature 1 later, there is disclosed a buried plug to close a bolt hole in a guide rail of a linear motion guide unit, in which the buried plug has a leading end having a height extending from a bottom of the guide rail across ⅕ the overall height of the guide rail. The buried plug, when placed in the bolt hole preparatory to press-in operation, may be held in proper attitude with stability by virtue of the leading end which has a peripheral circular surface smaller in diameter than the inside diameter of the bolt hole. This means that the buried plug at the leading end thereof is easily introduced into the bolt hole and kept in the bolt hole with a stable attitude. The buried plug while keeping the stable attitude is press fit or forced deep into the bolt hole at a mating zone. The mating zone has a plurality of relief grooves deeper than the interference for press fit. The power of the press fit is allowed to flee into the relief grooves so that the guide rail is protected against deformation such as bulging sidewise.
In a commonly-owned Japanese Laid-Open Patent Application No. 2007-192 282 there is disclosed a linear motion guide unit with rollers or balls, which is befitted for machines such as tool machines. The linear motion guide unit constructed as stated earlier has a buried plug which can fit easily and stably into a bolt hole. The buried plug is made of hard material including iron and stainless steel and rich in durability so as not to suffer to any damage caused by cutting chips. The buried plug is a ceiling plate which is made flush with the top surface of the guide rail so as not to cause damage to the end seal lips of the slider.
With the prior linear motion guide units in which the buried plug is press fit into a counterbored hole formed in the guide rail, as the guide rail is inevitably become less in thickness near the counterbored hole thereof, the guide rail, upon being press fit of the buried plug into the counterbored hole, bulges sidewise and particularly the races formed on both sides on the guide rail possibly suffer deformation or warp, which would cause obstacles to a smooth reciprocating movement of the slider along the guide rail. With the linear motion guide unit in which the slider moves on the guide rail by virtue of the rolling elements rolling through the races on the guide rail, thus, it remains a major challenge to keep the races on the both sides of the guide rail against the deformation to make sure of smooth sliding of the slider by virtue of the rolling elements interposed between the slider and the races on the guide rail.
With the prior linear motion guide unit constructed as stated earlier, the buried plug around the mating area thereof has a plurality of grooves to lessen an adverse impact which would be applied on the races at the time of press fit of the buried plug. Moreover, the prior buried plug recited earlier is made of aluminum alloy or brass, which is hard to give rise to falling away from the hole and collapse caused by the outside load, compared with the synthetic resin-made buried plug. Though the prior buried plug is contrived to restrain from the adverse impact on the races upon press fit of the buried plug into the counterbored hole in the guide rail, the buried plug has an interference extending around the full circumference of the counterbored hole. Namely, the load or stress caused by the interference with the bolt hole is allowed to free itself into the grooves around the mating zone and, in doing so, the buried plug is kept against excessively pushing the bolt hole away radially outwardly, so that the races on the guide rail is protected against outward bulge or deformation. Nevertheless, the prior buried plug constructed as stated earlier has the interference spreading uniformly over the full circumference of the bolt hole, so that the load or stress is exerted on the full circumference of the bolt hole, thereby probably negatively affecting any impact such as deformation on the race surfaces extending lengthwise of the guide rail. Near the bolt hole in the guide rail, moreover, as the thickness in width of the guide rail is thinner than in lengthwise of the guide rail, the buried plug when press fit into the counterbore of the bolt hole creates the load or stress trending to expand outwardly the counterbore in the guide rail, thereby somewhat bulging outwards the race surfaces on guide rail. In conventional machines, such slight deformation on the race surfaces has too little impact on the duration or traveling accuracy to cause trouble or questions. In the machine tools requiring severe operating conditions or machines demanding high traveling accuracy, it still remains a major challenge to develop a buried plug effective to restrain the deformation on the race surfaces of the guide rail.
Moreover, a buried plug envisaged using in the linear motion guide unit employed in the machines operating in severe environment has been developed. The buried plug is made of hard material such as iron, stainless steel, brass, ceramics or the like rich in duration, and hard to be damaged and worn. The buried plug has the grooves around the mating zone to make it possible to regulate fit in force in the bolt hole and make it easier to fit in the bolt hole. But, the buried plug in the linear motion guide unit constructed as stated earlier as having the interference extending uniformly around the full circumference of the counterbored hole, has the same issues as in the patent literature 1.